In general, an HVDC (High Voltage Direct Current) system converts an alternating current produced at a power plant into a direct current and transmits the direct current, and reconverts the direct current into an alternating current at a power reception terminal to supply power to a load. Such an HVDC system can efficiently and economically transmit power, connect different systems, and transmit power a long distance with high efficiency by increasing a voltage.
An MMC is connected to the HVDC system to transmit power and compensate for reactive power. Such an MMC includes a plurality of sub-modules connected in series to each other. The sub-modules are very important components in the MMC and there is a need for a power supply apparatus for stably supplying power to the sub-modules under various environments.
FIG. 1 is an equivalent circuit diagram of an MMC and FIG. 2 is a circuit diagram of a power supply apparatus for sub-modules of an MMC of the related art. As well known in the art, an MMC is composed of one or more phase modules 1 and a plurality of sub-modules are connected in series to each other in each of the phase modules 1. Further, the phase modules 1 each connect a DC voltage side to positive (+) and negative (−) DC voltage bus bars P and N. The input voltage of the P-N bus bars is input to the sub-modules (SM) 10 through connection terminals X1 and X2.
A power supply apparatus 20 for sub-modules of an MMC converts and change high-voltage (about 2˜3 kV) into low voltage (about 5˜100V) of the P-N bus bars to sub-modules in order to supply power for operation of the sub-modules. To this end, in the power supply apparatus 20 of the related art, the input voltage of the P-N bus bars of the MMC is increased to high voltage (for example, 3 kV) from 0V and Vdc voltage is stored in a capacitor 21. While the Vdc voltage that is the input voltage of the P-N bus bars is increased from 0V to 100V, clamping voltage Vzd of a Zener diode (ZD) 23 is output to a controller 24 and, the controller 24 receiving the clamping voltage Vzd turns on a semiconductor switch 25 so that current is supplied to a transformer 26. When secondary output voltage Pcon of the transformer 26 is applied to the controller 24, the controller 26 drives the power supply apparatus 20.
However, in this case, even though the input voltage Vdc is lower than the rated voltage of the power supply apparatus 10, the power supply apparatus 20 is started, but it stops without producing normal output due to low voltage in the early stage. Starting and stopping are continuously repeated while the input voltage is increased, and are stopped only when the input voltage reaches the rated voltage. When the input voltage reaches the rated voltage and the power supply apparatus 20 produces normal output, the secondary output voltage Pcon of the transformer 26 takes charge of power for the controller 24.
As described above, according to the related art, the power supply apparatus 20 is started even though input voltage is lower than rated voltage in the early stage, but it cannot normally operate. Further, even if the input voltage keeps increasing and reaches a high-voltage level, current keeps flows to a resistor 22 and a Zener diode 23, so these devices 22 and 23 generate heat, thereby keeping generating a loss.
Accordingly, it is required in the related art to develop a power supply apparatus that can remove unnecessary operations and reduce a loss in power supply apparatuses for sub-modules of an MMC that is connected with an HVDC system.